Electronic dispersion compensation has become a key feature in fiber-optic communication systems enabling larger tolerance to optical transmission impairments caused by chromatic and polarization mode dispersion effects. One of the most powerful techniques to electronic dispersion compensation is based on the Maximum-Likelihood-Sequence-Estimation (MLSE) algorithm. In practice, most MLSE implementations are based on the Viterbi algorithm.
The circuit implementation of the Viterbi algorithm at high data rates such as 10 Gb/s poses a particular challenge. The Viterbi algorithm relies on recursion that requires at-speed real-time computations that are extremely difficult to perform in a cycle time in the order of 100 ps (pico-seconds) while maintaining the accuracy required to achieve large dispersion tolerances paired with satisfactory bit error rate.
Existing techniques for high-speed MLSE electronics dispersion compensation are typically using an analog-to-digital converter front-end with most of the signal processing performed in the digital domain. Since current semiconductor technology does not allow at-speed computations with the required precision, the data out of the analog-to-digital converter is parallelized and rate-reduced. The standard Viterbi recursion loop is unfolded and a limited number of recursions are performed on a window of the data sequence in a massive parallel effort. This approach limits the accuracy to the achievable analog-to-digital converter speed/resolution and the length of the data sequence window.
The application for the current invention is an implementation of the Viterbi algorithm for optical fiber communications that is suited for at-speed implementation of the recursion loop by keeping the majority of the high-speed signal processing in the analog domain. This analog signal processing approach can yield improved accuracy and reduced circuit complexity making this technique suitable for integration into low-power pluggable optical transceiver modules while providing superior levels of dispersion compensation and optical receiver sensitivity.
One of the key aspects of achieving high analog accuracy is proper calibration of the analog signal processing circuitry. The current invention describes a calibration technique that overcomes the accuracy obstacles in an analog Viterbi recursion loop implementation.